S105 could be an engine number but I suspect that it could be a matching number. Where was the number stamped? If it was stamped on the back edge of the engine, next to the split of the 2 engine halfs then it is a matching cast number.
If the number is stamped into the front of the engine or the top of the drive side half (under the cylinder) then it is a 'S' model. (I thought the S model had a different engine but I'm not sure).
The model G in the link posted by LAB is a mixture of Model CO and G. The G had a taller headstock than that shown in the photo so must be a model CO front frame (maybe even back frame) and late CO girder forks.
Cheers, Chris.